Now that the PMU implementation is independent of HWPMC
as of r288992 use it to manage the CCNT.

Use the CNNT for get_cyclecount() instead of binuptime() when device pmu
is compiled in; if it fails to attach, fall back to the former method.

Enable by default for the BeagleBoneBlack configuration.

Optained from: Cambridge/L41
Sponsored by: DARPA/AFRL
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D3837